The influence of nano-size La₂O₃ and HfC on the microstructure and mechanical properties of tungsten alloys by microwave sintering

2.1 Composites preparation

This study uses commercial powders as raw materials, with the ratio of γ bonding phase Fe, Co, and Ni kept constant at 7:3:2 in all samples. Four alloy compositions were designed, and the sample names and mass fractions are listed in Table 1. The corresponding powders were mixed via ball milling according to the composition ratio, using tungsten carbide milling balls, with a ball-to-powder ratio of 10:1, a speed of 300 r/min, and a ball milling time of 6 h. Anhydrous ethanol was added as a process control agent during ball milling to suppress powder cold welding, prevent agglomeration, and control the milling temperature from becoming too high. To further increase the material’s density, this study employs a combination of extrusion and cold isostatic pressing to consolidate the powders. The freeze-dried composite powders were first held under a 250 MPa uniaxial pressure for 5 min to produce green bodies with certain strength and density. The green bodies were then densified using cold isostatic pressing technology to further enhance their density, this process was carried out through staged pressurization: In the first stage, a pressure of 100 MPa is maintained for 1 min, then it is increased to 200 MPa and hold in 1 min, and the third stage hold a 290 MPa pressure for 1 min. After that, reverse the process and reduce the pressure in the opposite order of the previous three stages. Finally, the green bodies were placed in a fiber cotton insulated container and sintered in a microwave sintering furnace under vacuum: The temperature was first increased to 1,400 °C at a rate of 40 °C/min, then raised to 1,470 °C at a rate of 10 °C/min, held for 30 min, and then cooled to room temperature with the furnace. The composite material was ultimately obtained. The preparation process is shown in Figure 1.

Figure 1:

Schematic diagram of the preparation process of WHA composites.

2.2 Characterization and test

Optical microscopy (OM) was used to observe the metallographic structure of the alloy and determine the preliminary morphology of the tungsten-based composite material. The phase composition and microstructure of the tungsten-based composite material were analyzed using X-ray diffraction (XRD, Rigaku Ultima IV) and scanning electron microscopy (SEM, ZEISS Sigma 300). The overall density of the tungsten-based composite material was measured using the Archimedes drainage method. Microhardness was measured using the HXD-100TM/LCD microhardness tester, with a fixed load of 10 kg and a holding time of 15 s. The mechanical properties were measured using the MTS E45.105 universal testing machine, with a loading rate of 0.00025 mm/s. The test specimens have a gauge length of 25 mm with the corresponding strain rate of 1 × 10⁻⁵ s⁻¹, which represents a typical lower limit of the quasi-static range. Finally, SEM was used to analyze the tensile fracture surface of the material to determine its fracture mode. Three repeated tests were conducted for each measurement to obtain the average and standard deviation, ensuring the reliability and accuracy of the data.

3.1 Phase analysis

The XRD spectrum shown in Figure 2 indicates that the WHAs are composed of a BCC-W matrix and an FCC-γ bonding phase. After the addition of La₂O₃ alone, its characteristic peaks were not observed in the spectrum, possibly due to insufficient diffraction effects preventing the La₂O₃ peaks from appearing [20]. When HfC was added alone, clear diffraction peaks for HfC and W-rich carbides appeared in the spectrum, indicating that the HfC particles are relatively large and well-crystallized, capable of pinning grain boundaries and refining the structure. When La₂O₃ and HfC were introduced in combination, the intensity of the HfC diffraction peaks are weakened and La₂O₃ was still not detected due to insufficient diffraction effects, indicating that the rare earth elements suppressed carbide growth during sintering, causing the particle size to decrease further or partially dissolve into the matrix.

Figure 2:

XRD images of WHA composites.

3.2 Microstructure analysis

The effect of different second-phase reinforcing particles (La₂O₃, HfC) on the microstructure of WHA matrix composites was analyzed using OM, and the results are shown in Figure 3. Figure 3a shows typical tungsten particles (bright white polygonal shape) evenly distributed in the gray bonding phase matrix. The overall structure is clear and dense, but a small number of micro-voids are also visible, which is a typical feature of the base WHA [18]. Figure 3b shows the addition of La₂O₃ particles, which are dispersed as small dark spots in the bonding phase and at the particle boundaries. This effectively refines the tungsten grains and significantly reduces the number of voids, indicating that La₂O₃ promotes densification and improves microstructure uniformity during sintering. In Figure 3c, although no distinct HfC particles were directly observed, possibly due to the low contrast with the matrix or the particle size being close to the resolution limit, the microstructure shows a more uniform distribution of tungsten grains and bonding phase morphology, indicating that HfC may improve structural consistency by inhibiting grain growth or promoting sintering. In Figure 3d, both La₂O₃ and HfC were added, and the presence of La₂O₃ particles can be identified. The bonding between tungsten particles is tighter, the voids are further reduced, and the microstructure uniformity is significantly better than that with a single addition, demonstrating the synergistic effect of the second-phase composite reinforcement. Comparing the microstructure of these four samples, the average sizes of tungsten particles are 6.2 μm (S-A), 7.7 μm (S-B), 14.1 μm (S-C), and 13.8 μm (S-D), respectively. The increased particle size and reduced micro-voids can be attributed to the promoting effect of La₂O₃ and HfC on the sintering process [3], 17]. On the other hand, La₂O₃ and HfC particles can provide more nucleation sites for tungsten particle growth, thereby refining the grains of the tungsten matrix. Therefore, the introduction of both La₂O₃ and HfC optimizes the microstructure of the WHAs, but since HfC is difficult to identify under OM, SEM in BSE mode and EDS are needed for further compositional and distribution characterization to clarify its form and strengthening mechanism.

Figure 3:

OM images of WHA composites. (a) S-A, (b) S-B, (c) S-C, and (d) S-D.

Figure 4a shows a SEM morphology of S-A in BSE mode, the bright white W particles are surrounded by a thin dark gray (Fe, Co, Ni) layer, but black micro-voids are still scattered at the three-phase boundaries, becoming the earliest stress concentration sources and crack initiation points under external forces [21]. The EDS surface scan in Figure 4b confirms that Fe, Co, and Ni form a continuous network in the gaps between W particles, ensuring complete alloying of the sintering neck. It both “welds” the hard phase and relaxes stress under load due to the high dislocation activity of the FCC structure. The point scan quantitative results in the table of Figure 4 show significant differences in the solubility of W in the bonding phase in different regions. The low W regions remain soft and ductile, which is beneficial for plastic deformation. The high W regions enhance lattice matching with W particles, increasing interface bonding strength, and the two complement each other, balancing both toughness and strength. The line scan in Figure 4c further reveals the W/bonding phase interface situation, where the W content rapidly decreases while Fe, Ni, and Co increase simultaneously. There are no hard and brittle intermetallic compounds at the interface, indicating that the bonding phase has fully wetted and tightly connected the W particles.

Figure 4:

(a) SEM image of S-A, (b) element distribution, and (c) line scan.

Figure 5 shows the microstructure of the S-B composite sample. Figure 5a shows that La₂O₃ particles preferentially distribute along the bonding layer, forming dispersed points that hinder dislocation motion and suppress grain boundary sliding during the later stages of sintering. This transforms the bonding phase from a “soft medium” into a “strong and tough framework,” providing the alloy with higher yield strength and fatigue resistance. The EDS mapping shown in Figure 5b confirms that La₂O₃ remains undecomposed, maintaining its complete crystal structure, and can exist stably for a long time, continuously purifying the grain boundaries and reducing the interface energy. The line scanning in Figure 5c further reveals that along the W-bonding phase-W path, the La peak only appears in the middle bonding region, with no La diffusion into the W matrix on either side. The interface transition is steep, and no brittle intermetallic compounds are formed. This indicates that La₂O₃ enhances the bonding phase strength and overall alloy toughness effectively through a dual mechanism of “pinning + purification” without affecting the interface bonding [22].

Figure 5:

(a) SEM image of S-B, (b) element distribution, and (c) line scan.

Figure 6 shows the SEM images in BSE mode of S-C (1 wt% HfC). The point scanning data in Figure 6a shows that the introduction of HfC promotes the formation of new carbides in the matrix. Combined with the EDS analysis in Figure 6b, it is inferred that these newly formed carbides are (Hf,W)C [23], which is also confirmed by the XRD analysis (Figure 2). This phenomenon is attributed to the localized high-temperature regions generated inside the powder during microwave sintering, which significantly increases the carbon diffusion coefficient, making it easier for carbon in HfC to release into the matrix. Although Fe, Co, and Ni are bonding elements, under high carbon activity and high-temperature conditions, they can combine with W and C to form composite carbides. The rapid heating characteristics of microwave heating suppress the formation of equilibrium phases, allowing these metastable carbides to be retained. The formation of these composite carbides helps to improve the hardness and wear resistance of the alloy. In addition, a small amount of HfC phase remains in the material. The line scan results in Figure 6c reveal that these HfC particles are primarily distributed around the bonding phase, that is, between the tungsten particles.

Figure 6:

(a) SEM image of S-C, (b) element distribution, and (c) line scan.

Figure 7 shows the SEM images of the sample after the addition of La₂O₃ and HfC (S-D). From Figure 7a, it can be observed that the composite carbides, similar to those in Figure 6, still exist, which is a result of the interaction between HfC and the matrix material. The point 3 elemental content analysis and XRD results in Figure 7b further confirm the existence of these composite carbides. In addition, the elemental content analysis of point 1 and the elemental distribution map in Figure 7b2 reveal that La₂O₃ and HfC tend to combine together and are mainly distributed in the bonding phase region. This distribution may be due to the interaction between La₂O₃ and HfC during the material preparation process, as well as their solubility and diffusion behavior in the bonding phase.

Figure 7:

(a, b) SEM image of S-D, (b₁, b₂) element distribution.

3.3 Relative density analysis

The effect of different second-phase reinforcing particles on the relative density of WHA matrix composites is analyzed, and the results are shown in Figure 8. The relative densities of all materials are at a high level, indicating that the sintering process achieved good densification. Specifically, the addition of La₂O₃ or HfC alone helps to further improve the material’s density, with relative densities of 99.1 ± 0.75 % and 99.2 ± 0.47 %, respectively. They are higher than the SPS prepared W–Ni–Fe–La₂O₃ composites (≤87.95 %) [24] and the 90W–7Ni–3Cu alloy prepared by conventional sintering [19]. This result suggests that La₂O₃ and HfC may effectively enhance the material’s sintering activity and densification through mechanisms such as inhibiting abnormal grain growth of tungsten, promoting diffusion mass transfer, or optimizing liquid phase distribution during sintering. It is worth noting that when La₂O₃ and HfC are added in combination, the relative density slightly decreases to 98.6 ± 0.73 %, but still remains at a high level. This phenomenon may be due to the interaction between the two reinforcing phases during sintering, such as local agglomeration between particles, interface hindrance of material migration, or differences in sintering kinetics, which slightly inhibit the final densification. Despite the slight decrease, the value remains within the error range, indicating that the overall densification ability of the composite addition system is still excellent.

Figure 8:

The relative density of WHA composites.

3.4 Mechanical properties analysis

By analyzing the Vickers hardness test results, the influence of different second-phase reinforcing particles on the strengthening effect of WHAs can be clearly revealed (Figure 9). Compared to the unreinforced pure WHA matrix, the introduction of second-phase particles significantly improves the material’s hardness, although it is higher than some of the reported W–Ni–Fe–La₂O₃ composites [24]. The strengthening mechanisms and effects of various second-phase particles are different. La₂O₃ particles mainly function through grain refinement and dispersion strengthening mechanisms. Their small oxide particles purify and pin the grain boundaries, hindering dislocation motion, thus enhancing resistance to plastic deformation [25]. In Figure 6, it was found that HfC undergoes mutual dissolution and reaction with the tungsten matrix, forming composite carbide phases. The formation of this new phase helps to improve the material’s hardness. On one hand, the in situ generated composite carbide phase has a higher interface bonding strength with the tungsten matrix than the added second-phase particles, effectively pinning dislocations and hindering grain boundary sliding, thus contributing a considerable dispersion strengthening effect. On the other hand, the unreacted HfC, as a high-hardness carbide phase, can effectively hinder dislocation motion in the tungsten matrix, leading to an increase in hardness [25]. The most significant effect is observed in the sample with the composite addition of La₂O₃ and HfC, where the hardness value reaches the highest level among all groups. This is attributed to the excellent synergistic strengthening effect produced by the two reinforcing particles. La₂O₃ mainly acts on grain refinement and grain boundary strengthening, while HfC provides strong dispersion strengthening due to the in situ reaction. The two mechanisms work together to more effectively increase the resistance to dislocation motion, resulting in the composite material exhibiting the highest hardness at the macroscopic level.

Figure 9:

The Vickers hardness of WHA composites.

Based on the tensile stress–strain curve of the WHA matrix composites (Figure 10), the strengthening and toughening mechanisms and effects of different added phases can be clearly revealed. From the performance benchmark of the base alloy S-A (W+(Fe, Co, Ni)), its tensile stress is 613.5 MPa, and its strain is only 0.9 %, which is higher than some WHAs [24], but it still demonstrates the typical high-strength and low-toughness characteristics of traditional WHAs, with clear brittle fracture behavior. After replacing part of the bonding phase with La₂O₃ in sample S-B, the performance underwent a qualitative leap, with its stress increasing to 727.6 MPa and strain significantly rising to 5.1 %. This result is mainly attributed to the dispersion strengthening effect of La₂O₃ oxide and its optimization effect on the microstructure. La₂O₃ particles effectively hinder dislocation motion, while refining the grains, purifying, and strengthening the grain boundaries and phase interfaces, thereby significantly enhancing strength while greatly improving the material’s plastic deformation ability, achieving excellent strengthening and toughening. In contrast, after replacing with HfC in sample S-C, its stress increased to 675.6 MPa, and strain only slightly increased to 2.1 %. This indicates that HfC, as a hard second phase, mainly improves strength through second-phase strengthening mechanisms, but its inherent brittleness and metastable carbides somewhat limit further improvement in toughness, making its strengthening and toughening effect less significant than La₂O₃. In sample S-D, the simultaneous introduction of La₂O₃ and HfC resulted in synergistic optimization of performance, achieving high strength (744.1 MPa) and toughness (4.9 %). This indicates that the two additives played an ideal complementary role. HfC, as the main strengthening phase, provides the foundation for strength, while La₂O₃, while contributing to dispersion strengthening, more critically counteracts the brittleness risk that HfC may introduce through its toughening mechanism, together enhancing the overall performance of the material. In summary, the variation pattern of WHA samples reveals that La₂O₃ is an effective toughening agent in WHAs, while HfC is a reliable strengthening agent. Their combination can achieve a balance of performance. Therefore, through multiscale microstructure design, which synergistically utilizes different types of second phases to overcome the strength-toughness contradiction of WHA, a clear theoretical basis and practical path are provided. It is worth noting that the stress–strain curve shows a significant difference in slope before and after the inflection point (yield point), and the fundamental reason lies in the fundamental change in the dominant deformation mechanism of the material. Before the inflection point, reversible elastic deformation occurs due to the cooperative interaction of all atoms, showing a constant high slope (elastic modulus). After the inflection point, irreversible plastic deformation dominated by dislocation slip, proliferation, etc. begins, and its slope (work hardening rate) strongly depends on the ability of second-phase particles, grain boundaries, and other defects to hinder dislocation motion.

Figure 10:

The tensile stress–strain curve of WHA composites.

3.5 Fracture analysis

To analyze the strengthening mechanism, fracture surface characterization was performed on each composite. Figure 11a shows that the fracture mode of the WHA is intergranular fracture. From the magnified images in Figure 11b, 11b1, and 11c, it can be seen that the γ bonding phase is distributed between W grains, acting as a bridge, which helps to improve the alloy’s toughness. In addition, it can be seen that micropores are mainly distributed in the bonding phase, which may lead to premature failure of the bonding phase, preventing it from functioning effectively. Figure 11a indicates that the WHA primarily exhibits intergranular fracture. Figure 11b and 11b1 show that the γ bonding phase forms a continuous thin-film coating around the W grains. During crack propagation, it undergoes noticeable plastic deformation and forms bridging ligaments, effectively suppressing crack instability and improving alloy toughness. As seen in Figure 11c, the micropores are mainly distributed in the aggregated bonding phase, becoming the sites of initial failure. This occurs because the micropores preferentially nucleate, grow, and link within the bonding phase, leading to local stress concentration. When the void volume fraction reaches a critical value, the bonding phase fails prematurely, the bridging effect decreases sharply, and it becomes the limiting factor for toughness reduction.

Figure 11:

(a, b, c) Fracture surface morphology of S-A, (b₁) element distribution in b.

Figure 12 reveals the regulatory effect of La₂O₃ on the fracture behavior of WHAs. When La₂O₃ is not added, cracks rapidly propagate along the W grain boundaries, showing typical intergranular brittle fracture (Figure 11). After adding La₂O₃, the fracture mode shifts to transgranular fracture (Figure 12a), indicating that the cracks are forced to cut through the W grains. The appearance of transgranular fracture means that the bonding strength of the grain boundaries is greater than the intragranular strength. The grain boundaries no longer serve as crack propagation paths, and more energy is required to tear the grains, which macroscopically manifests as improved impact toughness and fracture toughness. Figure 12b and 12b1 show that La₂O₃ and the γ bonding phase are distributed between the W grain boundaries, which helps to improve the bonding strength between the W grain boundaries, thereby increasing the strength of the composite material. Therefore, the addition of La₂O₃ increases the bonding strength of the W grain boundaries, making it greater than the intragranular strength, causing the fracture mode to transition from intergranular to transgranular, which macroscopically results in increased toughness and strength. La in La₂O₃ has high activity and preferentially combines with harmful impurities such as O, S, and P enriched at the grain boundaries, eliminating impurity embrittlement. At the same time, La₂O₃ segregates at the grain boundaries, reducing interface energy and hindering W grain boundary diffusion, significantly improving the grain boundary bonding strength after sintering [26]. In addition, La₂O₃ precipitates uniformly within the γ bonding phase (Figure 12c), increasing the strength of the bonding phase. After the bonding phase hardens, it more effectively transfers the load to the W grains, indirectly enhancing the load-bearing capacity of the grain boundaries. When the external load exceeds the cohesive strength of the grains, cracks can only propagate transgranularly, eventually leaving a cleavage appearance on the fracture surface, achieving an improvement in strength and toughness. Furthermore, La₂O₃, as a second-phase particle, can improve the composite material’s performance through the typical Orowan strengthening mechanism.

Figure 12:

(a, b, c) Fracture surface morphology of S-B, (b₁) element distribution in b.

As shown in Figure 13, after the addition of HfC, the fracture morphology of the composite material is characterized primarily by transgranular fracture (Figure 13a). According to Figure 13b and the corresponding EDS analysis results (Figure 13b1), the γ bonding phase is still distributed between W grains, serving as a toughening connector and effectively alleviating stress concentration. HfC particles are dispersed within the γ phase, significantly enhancing the load-bearing capacity of the bonding phase, thereby increasing the overall strength of the material. Through point scan analysis in Figure 13c, it can be seen that the transgranular fracture surface is enriched with W carbides, which is due to the introduction of HfC promoting the formation of composite carbides. These carbides are thermodynamically in a metastable state and under external loading, they are prone to becoming preferred sites for crack initiation and propagation, leading the fracture path to traverse through the grain interiors, exhibiting a typical transgranular fracture mode [27]. In addition, HfC particles can effectively hinder the crack propagation path during crack expansion, consuming fracture energy through pinning or deflection mechanisms, thereby enhancing the material’s fracture toughness and high-temperature performance. In summary, the addition of HfC improves the mechanical behavior of the composite material by regulating the performance of the bonding phase and inducing second-phase strengthening.

Figure 13:

(a, b, c) Fracture surface morphology of S-C, (b₁) element distribution in b.

Figure 14 shows the SEM images of the WHA matrix composite synergistically reinforced by La₂O₃ and HfC. As seen in Figure 14a, the fracture mode of the composite material is a mixed pattern of transgranular and intergranular fracture. The γ bonding phase is uniformly distributed between the W particles, forming a continuous network structure that significantly enhances the material’s toughness. La₂O₃ and HfC are mainly distributed in the grain boundary regions (Figure 14c and 14b1), where they not only purify grain boundaries and suppress impurity segregation but also effectively strengthen the γ bonding phase. During crack propagation, they promote crack deflection, consuming more fracture energy and thereby enhancing the overall strength and toughness. The synergistic effect of the two phases significantly optimizes the microstructure and mechanical properties of the composite material.

Figure 14:

(a, b, c) Fracture surface morphology of S-D, (b₁) element distribution in b.

In summary, based on the microstructure and fracture analysis, S-A (W+(Fe, Co, Ni)) represents the traditional strengthening path, where the strength mainly comes from the deformation resistance of the tungsten grains themselves and the high density achieved through liquid-phase sintering. The solid solution strengthening effect provided by the bonding phase is limited. This mechanism, due to its coarse grains and weak W-bonding phase interface, results in both strength and toughness being at lower levels. The S-B sample, with La₂O₃ introduced, has its strengthening mechanism centered around dispersion strengthening and grain boundary engineering. La₂O₃ particles, as stable obstacles, pin dislocations through the Orowan mechanism, thereby increasing strength. More importantly, it greatly enhances the interface bonding strength by inhibiting grain boundary migration, refining grains, and purifying the grain boundaries [28]. The S-C sample (with added HfC) mainly reflects the typical second-phase strengthening. HfC, as a hard particle, also enhances strength by hindering dislocation movement. However, due to the metastable carbide formed with the matrix, it easily becomes a nucleation site for microcracks during deformation, making its strengthening path more focused on a single increase in strength, with limited contribution to improving toughness [29]. The final S-D sample (with both La₂O₃ and HfC added) demonstrates a multiscale synergistic strengthening approach. HfC, as the main load-bearing phase, provides the basic strength increment, while La₂O₃ optimizes the interface and passivates cracks, effectively offsetting the brittleness risk that HfC may introduce. The two phases collaborate at different scales, jointly creating a composite material system that efficiently resists deformation and effectively dissipates fracture energy, thus achieving a balance between strength and toughness.